1. Field of the Invention
This invention relates to fuel cells and more particularly to electrolyte retaining matrices and methods for making the same.
2. Description of the Prior Art
Fuel cells for the production of electrical energy from a fuel and oxidant are well known in the art. Such cells in their most simplified design, comprise a housing, an oxidizing electrode spaced apart from a fuel electrode, and an electrolyte disposed between and in contact with said electrodes. The electrolyte can be a solid, a molten paste, a free-flowing liquid, or a liquid trapped in a matrix. This application is concerned with the latter type of matrix which is preferred for many applications.
For optimum performance in a fuel cell employing a trapped aqueous electrolyte, the matrix must exhibit certain properties. For example, the matrix must be hydrophilic. Also, it must be continuous to prevent gas crossover or mixing of reactant gases in the fuel cell; in other words, it should be entirely free from pin holes and cracks. It should be as thin as possible in order that the internal resistance losses through the electrolyte will be minimal. Intimate contact between the matrix and electrode surface is necessary to maximize catalyst utilization. Uniform thickness is also critical to good performance in that lack of uniformity can cause current maldistributions with a loss in performance. It is also desirable that the pore size distribution of the matrix be very well controlled so as to prevent gas crossover and to insure proper electrolyte distribution throughout the cell.
Compounding the problems of achieving the foregoing properties is the fact that one is limited in the choice of materials which can be used. For example, the materials must be chemically and thermally stable at cell operating temperatures; also, they must not poison the catalyst and they must have high electronic resistance. Finally, the matrix should be made by an economical process.
A common prior art economical method for making matrices has been by paper making techniques, wherein the matrix is formed into a sheet and sandwiched between the electrodes in a fuel cell or fuel cell stack by mechanical means. For example, Landi U.S. Pat. No. 3,407,249 forms sheets of fibrillated polytetrafluoroethylene. Mesite et al U.S. Pat. No. 3,627,859 forms a matrix sheet from cellulosic fibers in combination with a fluorocarbon polymer. Emanuelson et al U.S. Pat. No. 3,694,310 forms mats of matrix material from phenolic resin fibers coated with a phenolic beater addition resin.
Regardless of the material from which the mat is made, the mechanical sandwiching of a sheet type matrix between electrodes is deficient in that it does not necessarily result in intimate contact between the matrix and the electrode over the entire surface of the matrix. A further problem with making matrices by paper making techniques is that the desired thinness cannot be achieved without losing the property which prevents gas crossover. Even if the matrix sheet could be made as thin as desirable, it would be extremely difficult, if not impossible, to handle.
Another method for forming a matrix, which overcomes some of the problems with the paper making techniques, is to form the matrix directly on the surface of the electrode such as by dipping the electrode into an aqueous solution of the matrix material as described in Blanc et al U.S. Pat. No 3,022,244. This has also been accomplished by spraying or painting the matrix onto the surface of the electrode. While these techniques overcome some of the handling problems associated with separate matrix sheets, it is difficult to maintain a uniform thickness. Because of the nonuniformity of the thickness it may be necessary that some areas be thicker than desirable in order to assure that there are no bare spots in the thinnest areas.
Commonly owned U.S. Pat. Nos. 4,000,006 and 4,001,042 teach the use of screen printing for applying an electrolyte matrix to the surface of an electrode. Although screen printing overcomes virtually all of the problems discussed above with regard to other methods for making matrices, the speed of the process would probably be limited to the equivalent of about 100 feet of electrode length per minute for a fully automated operation. This is quite fast compared to other prior art methods, but it would be very desirable to be able to produce matrices at a considerably faster rate of speed. Also, despite their improvement over the prior art, it has been observed that matrices applied by the screen printing process show a pattern of "hills and valleys" which result from the imprint of the screen on the surface of the layer. If a similar quality matrix layer could be applied by a method which did not produce these "hills and valleys" (i.e., an even more uniform matrix layer) further advantages would be realized.
A well-known technique for applying thin layers of adhesives, lacquers or waxes on wood or composite material panels is "curtain coating". Chocolate and other coatings or candy and baked goods are also applied by the method of curtain coating. The curtain coating technique is simple in principle. Basically, a continuous, vertical curtain of the coating material is created by having the material flow through a narrow slot. A conveyor system carries the material to be coated through the curtain. The coating thickness is controlled, for the most part, by the conveyor speed, the slot width, the viscosity of the coating fluid, and the magnitude of the pressure which forces the fluid through the slot. Conveyor speeds of 500 feet per minute and higher are possible for the foregoing applications. All of the coating material which is not applied to the substrate being coated flows to a return trough and is reused by pumping it back to the slot. Additional details concerning curtain coating equipment for these prior art applications is found in a paper presented at the 1966 Annual Fall Seminar of the Adhesive and Sealant Council titled "The Use of Curtain Coating Equipment in the Application of Adhesives" by Dr. Charles E. Wetzler, said paper being incorporated herein by reference.
Despite the extensive use of curtain coating for the application of finishes and adhesives to a variety of substrates, there is no teaching or suggestion that a fuel cell matrix with all its attendant high technology properties and characteristics can be satisfactorily formed by the curtain coating process.